U-series disequilibria of trachyandesites from minor volcanic centers in the Central Andes

Fang Huang, Erik V. Sørensen, Paul M. Holm, Zhao Feng Zhang, Craig C. Lundstrom

Research output: Contribution to journalArticlepeer-review

Abstract

Young trachyandesite lavas from minor volcanic centers in the Central Andes record the magma differentiation processes at the base of the lower continental crust. Here we report U-series disequilibrium data for the historical lavas from the Andagua Valley in Southern Peru to define the time-scale and processes of magmatism from melting in the mantle wedge to differentiation in the crust. The Andagua lavas show (230Th)/(238U), (231Pa)/(235U), and (226Ra)/(230Th) above unity except for one more evolved lava with 230Th depletion likely owing to fractional crystallization of accessory minerals. The 226Ra excess indicates that the time elapsed since magma emplacement and differentiation in the deep crust is within 8000 years. Based on the correlations of U-series disequilibria with SiO2 content and ratios of incompatible elements, we argue that the Andagua lavas were produced by mixing of fresh mantle-derived magma with felsic melt of earlier emplaced basalts in the deep crust. Because of the lack of sediment in the Chile-Peru trench, there is no direct link of recycled slabs with 230Th and 231Pa excesses in the Andagua lavas. Instead, 230Th and 231Pa excesses are better explained by in-growth melting in the upper mantle followed by magma differentiation in the crust. Such processes also produced the 226Ra excess and the positive correlations among (226Ra)/(230Th), Sr/Th, and Ba/Th in the Andagua lavas. The time-scale of mantle wedge melting should be close to the half-life of 231Pa (ca. 33 ka), while it takes less than a few thousand years for magma differentiation to form intermediate volcanic rocks at a convergent margin.

Original languageEnglish (US)
Pages (from-to)92-104
Number of pages13
JournalGeochimica et Cosmochimica Acta
Volume215
DOIs
StatePublished - Oct 15 2017

Keywords

  • Andes
  • Central volcanic zone
  • Continental crust
  • Magma differentiation
  • U-series disequilibria

ASJC Scopus subject areas

  • Geochemistry and Petrology

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